The invention relates to an operations vessel for ice covered seas, and, more particularly, to a monopod structure having ice disaggregation system employing two fundametal modes of ice removal, each such ice removal mode being adapted for ice disaggregation in both a transit and operational phase of the vessel.
In the petroleum exploration and production industry it is often necessary to move and station men and equipment in relatively hostile environmental regions. In recent years the emphasis on oil production from the far north has necessitated development of new techniques for encountering formations of encroaching ice floes and the movements thereof which threaten the stability and/or position of equipment situated therearound.
In the Arctic, large offshore regions are often covered by thick layers of ice. Currently there is considerable activity in this and other frozen areas directed toward the location and development of sources of petroleum and other natural resources. The search for and production of these resources require operational platforms for locating equipment and personnel. These platforms are normally transported to their operational sites and maintained in a relatively fixed position with respect to the underwater floor by anchoring thereto and/or the utilization of dynamic positioning techniques. In the normal course of operation, pipes are extended from the platform into the earth's subsurface for the recovery of natural resources. It is then important to maintain the platform within a predetermined envelope in order to prevent breaking or withdrawing the pipe from the earth.
Platforms located in both shallow and deep covered waters are exposed to ice floes which sometimes float freely on the water. The ice may be comprised of such mass that a platform is susceptible to damage or destruction as a result of forces imparted thereagainst by the moving ice. The Arctic ocean, for example, is characterized by air temperature ranging from -70.degree. to 70.degree. F. and ice sheets of thickness between 6 and 10 feet and pressure ridges of 10 to 100 feet. In such conditions, ice normally exhibits a compressive strength of 1000-3000 PSI and tensile strength of 300-1000 PSI. The problems of providing the requisite magnitude of force and power necessary for engagement with and disaggregation of such an environmental threat may be seen to be formidable.
One prior art approach to drilling platforms for ice covered seas has previously included a monopod, semi-submersible drilling platform design utilizing a single rotating cutter for ice floe engagement and disaggregation. The cutter is disposed between an upper superstructure comprising an operation platform and a submerged hull providing flotation. In this manner, only a relatively narrow profile emerges through encroaching ice layers while maximized platform surface area and buoyancy size parameters are met, respectively above and below the ice. This concept has been theoretically effective although a plurality of feasibility problems plague its realization. For example, a single rotating cutter capable of disaggregating such enormous masses of ice would produce a torquing about the submerged hull which would consume large quantities of fuel to counteract. Fuel must be stored aboard the platform which requires a storage area formed of heavy steel and iron, and this additional weight and bulk further increases the fuel requirements for moving the platform and stabilizing it above a borehole or within a predetermined envelope. Fuel necessary for counteracting torque may thus be equated to size and cost in a spiraling relationship. Size and cost are also primary factors in the transit phase of the platform operation and the problems related thereto have proven to be as formidable as the ability to stabilize the platform against ice movement.
It would be an advantage therefore, to overcome many of the disadvantages of prior art structures by providing an operations platform for ice covered seas adapted for transit movement therethrough and effective stabilization therein. The apparatus of the present invention is provided for just such a purpose wherein a monopod flotation structure is constructed with an intermediate hull comprising an ice breaking bow and ice disaggregating stern. The intermediate hull is similarly constructed for presenting a narrow profile to encountered ice floes. However, a pair of ice chipping and breaking drums are also provided for counter-rotation in order to disaggregate encroaching ice while in either the transit or operational mode. The paired configuration negates resultant torques. Moreover, the cutters are housed aft of a bow construction facilitating movement through both ice covered and open waters as is necessary for positioning of the platform above a selected functional operating region or in other operational modes as may be developed.